1. Field of the Invention
This invention relates to digital-to-analog converters (DACs), and more particularly to voltage-scaling DACs.
2. Description of the Prior Art
The numerous types of DACs that are presently available can generally be categorized as current-scaling, voltage-scaling or charge-scaling devices. A general discussion of DACs is provided in Grebene, Bipolar and MOS Analog Integrated Circuit Design, John Wiley & Sons, 1984, pages 753-824.
Voltage-scaling DACs produce an analog output voltage by selectively tapping a voltage-divider resistor string connected between high and low reference voltages, with the low reference generally being set at ground. These types of converters are used most commonly as building blocks in MOS analog-to-digital conversion systems, where they function as the DAC subsection of a successive-approximation-type analog-to-digital converter. For an N-bit voltage-scaling DAC, the resistor string consists of 2.sup.N identical resistors connected in series, and is used as a potentiometer in which the voltage levels between successive resistors are sampled by means of binary switches. Replacing mechanical potentiometers and rheostats is an important and potentially very high volume application for these devices.
FIG. 1 is a schematic diagram of an N-bit DAC that operates on the voltage-scaling principle. A resistor string consisting of resistors R1, R2, R3, . . . , R2.sup.N -1, R2.sup.N is connected between a high reference voltage (VREF+) node 2 and a low reference voltage (VREF-) node 4, which are typically 5 volts and ground potential, respectively. The voltage drop across each resistor is equal to one least significant bit (LSB) of output voltage change. The output is sampled by a decoding switch network, illustrated as switches S1, S2, S3, . . . , S2.sup.N. Each switch taps a different point in the resistor string, so that closing a particular switch while leaving the other switches open places a unique analog voltage on a common output line 6 to which each of the switches is connected. A decoder (not shown) controls the operation of the switches so that the switch whose voltage corresponds to the magnitude of the input digital signal is closed. The signal on analog output line 6 is sensed by a high-impedance buffer amplifier or voltage follower A1, the output of which is connected to an output terminal 8 that provides the final output analog voltage. To ensure the accuracy of the conversion, the buffer amplifier should draw negligible DC bias current compared to the current within the resistor string.
A principal drawback of this type of circuit for high-bit-count D/A conversions is the very large number of components required: 2.sup.N resistors, 2.sup.N switches and 2.sup.N logic drive lines. For example, in a 12-bit implementation, this approach would use 4,096 resistors, 4,096 switches and 4,096 logic drive lines. It would be highly desirable to significantly reduce this large number of elements for purposes of area savings, higher manufacturing yields and lower costs.
Voltage-scaling DACs are presently available which greatly reduce the number of required resistors and switches by using one resistor string consisting of 2.sup.N/2 resistors for the input digital signal's most significant bits (MSBs), and a separate resistor string also consisting of 2.sup.N/2 resistors for the LSBs. Each resistor in the LSB string has a resistance value equal to 1/2.sup.N/2 the resistance of each MSB resistor. The opposite ends of the LSB string are connected across one of the MSB resistors. By varying the MSB resistor selected for the LSB string connection and taking an output from the LSB string, outputs in one LSB increments can be obtained over the full range of one to 2.sup.N/2 -1 LSBs. Two such circuits are the AD569 and AD7846 DACs by Analog Devices, Inc., described in the Analog Devices 1992 Data Converter Reference Manual, Vol. 1, pages 2-83 to 2-94 and 2-721 to 2-734. However, to preserve the constant resistance characteristic of the MSB string, active amplifier buffer circuits must be used to interface between the MSB string and its connection to the LSB string. This unfortunately makes the device unusable for potentiometer and rheostat purposes.